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Area forecast discussion for southeast South Dakota/southwest Minnesota/northwest Iowa 
National Weather Service Sioux Falls South Dakota 
1110 am CST Monday Nov 23 2009 


Discussion... 
cloudy day across the County Warning Area...especially ahead of the inverted trough. 
This trough is beginning to slowly work east with winds swinging 
around to the northwest. Upper level wave per the water vapor loop 
shows it moving into Nebraska...which should continue to transition 
eastward today. Latest model data indicate that as this wave moves 
east...any interaction with the middle level boundary currently 
producing precipitation in western Nebraska will weaken...as the 
wave encounters a stronger...lower level front with some instability 
over eastern Nebraska into northwest Iowa. Cross section of the 
front and instability do suggest that something around the 800-750mb 
layer would be most supportive of precipitation late this afternoon 
and early this evening and saturation/moisture does indicate that 
some decent precipitation amounts will be possible. Best chance for 
heavier rainfall should be east of a Vermillion to Slayton line. 
Otherwise...with clouds do not expect much change in temperatures 
through the day...so going forecast looks good. /08 




&& 


Aviation... 
stratus along and east of an incoming inverted trough...mainly along 
and east of Interstate 29... will continue through the afternoon. 
Occasional IFR ceilings will be possible with isolated IFR 
visbys...although most of the visibility threat should be greatly 
diminished. As this trough passes the threat for IFR conditions 
decreases and expect occasional VFR to MVFR conditions through the 
afternoon west of Interstate 29. By late afternoon a band of 
rainfall should develop from about Vermillion to Slayton Iowa and 
shift eastward through the night. Some of this rain could be 
moderate to occasionally heavy with MVFR conditions. 


&& 


Previous discussion... 
two systems to contend with through the middle of the week. For the 
first one today through Tuesday night...the NAM appeared to be an 
outlier in taking the upper dynamics too far south when compared to 
the GFS/European model (ecmwf) and Gem regional. The latter three models were all 
very close with their respective placements of the upper dynamics 
and associated surface features through Tuesday night...abeit the Gem may be 
a bit too fast in ejecting the system eastward on Tuesday/Tuesday night. 
Therefore it appears the crux of the upper level energy will track 
tonight in extreme northern Kansas...then along the Iowa/MO border on 
Tuesday...then into northern Illinois Tuesday night. Kind of a nuisance day today. 
Patchy fog will be found this morning in our western forecast area...which was west 
of the original main mass of stratocu. In fact...with some clearing 
in our SW County warning forecast area early this morning...some radiational dense fog will be 
found in areas around Charles Mix...Bon home...Douglas and Davison 
counties. Accounted for this in the forecast. Otherwise things get a 
little bit more organized this afternoon with a deeper moisture 
profile and the slow passage of the surface inverted trough. Therefore 
measurable probability of precipitation will be higher in the eastern half of our forecast area this 
afternoon...with some likely probability of precipitation warranted right along the low 
level frontogenesis maximum near the inverted trough in SW Minnesota and much 
of northwest Iowa. Maximum temperatures through the eastern half of our forecast area will not move a whole 
lot due to the cloud cover and poor mixing. Am hoping for a few 
breaks in the clouds this afternoon from Tyndall South Dakota to Huron South Dakota and 
points westward for their maximum temperatures. Otherwise if not...the forecast maximum 
temperatures for them will be too warm. But mixing potential at least on the 
forecast soundings is higher in our west when compared to the eastern half. 


The tonight and Tuesday morning period continue to be the main focus of the 
system. Deep moisture entrains in ahead and to the north of the surface 
and upper lows...then begins to rapidly dry out heading westward toward 
Mitchell and Huron and points westward. Warm air advection and isentropic lift are 
rather weak though...with virtually no trowaling noted. In 
addition...cold air is not wrapping into this system which should 
keep snowfall on the backside to a minimum. Once the thermal 
profiles cool off enough for snow...it is also coincided with an 
onslaught of drier air advecting in from the northwest. Therefore a big bulk 
of this system will be rain for measurable precipitation. Despite overall weak 
dynamics...areas in SW Minnesota and northwest Iowa are modestly placed in the left 
exit region of the jet streak to the south...coupled with deep 
moisture lasting for quite some time. Therefore 0.75 to 1.25 inch 
rain amounts could occur in much of SW Minnesota and northwest Iowa tonight through 
Tuesday. Made little changes to temperatures tonight and Tuesday. With less 
clouds...there should be an actual diurnal rebound in our western forecast area on 
Tuesday. But temperature movement will be minimal in our eastern half on Tuesday 
off of lows. 


Further out...all of the various models are in good agreement in 
handling the polar air mass moving southward with the next wave diving down 
from western Canada. The actual surface front moves southeastward Tuesday night...setting 
up a windy day on Wednesday. Tuesday night...there could be some lingering 
light rain in our far eastern areas in the evening. This will then be 
replaced by very small chances for -sn in our northern areas behind the surface 
front late Tuesday night. Went cooler then guidance readings on Wednesday as 
the stratocu field looks intense except for maybe Gregory County. 
But there should be enough mixing with the strong northwest winds to move 
about 5 to 10 degrees off of Wednesday mornings low temperatures. That being said... 
Wednesday will not be a pleasant day by any stretch of the imagination. 
Light snow showers will be around in the cold air advection except for our SW. In 
time...we will have to watch SW Minnesota as probability of precipitation may have to be raised in 
that area in future forecasts for some light measurable snowfall which 
could stick. And with the wind forecast...little concerned about blsn in 
that area...again on Wednesday. A little bit of -sn may linger in our east 
Wednesday night...but I may be too slow in ejecting it eastward. 


After words...upper ridging begins to takeover for the remainder of 
the week giving dry conds again. /Mjf 


&& 


Fsd watches/warnings/advisories... 
South Dakota...none. 
Minnesota...none. 
Iowa...none. 
NE...none. 


$$ 

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